Arch shape cross section commercial thermoplastic storm chambers are familiar in commerce. They have been made by injection molding and thermoforming. Before more tailored products were developed, wastewater leaching chambers had been used as storm chambers. Typically, an interconnected array of chambers is buried within permeable soil to create large void spaces. Stormwater, such as results from rainfall on a paved parking lot, is flowed to the chambers. The water is detained, and over time either controllably flowed to a discharge point, and or allowed to dissipate through the earth.
A type of chamber relevant to the present invention has a curved arch shape cross section and spaced apart crest corrugations and valley corrugations running transverse to the length. (Crest corrugations have been referred to as peak corrugations in numerous patents relating to chambers.) The corrugations strengthen the chamber and are differentiated from what is called ribs or ribbing, which is the name given to relatively narrow plastic structures, also used for strengthening, and often found running lengthwise. See U.S. Pat. No. 5,716,163 of Nichols et al. for information about ribbing.
Prior art commercial storm chambers have had various sizes. Smaller chambers have been about 3 feet wide and 8-10 feet long. The SC-310 chamber and SC-740 chamber of Stormtech LLC, Wethersfield, Conn., exemplify current chambers. As an example, the SC-740 chamber is about 85 inches long, 51 inches wide and 30 inches high, and weighs about 74 pounds.
There has been market place opportunity for larger dimension chambers in the belief they would provide more favorable cost per unit volume of water contained within the chamber, and a smaller footprint for a given capacity stormwater system. Any new large chamber desirably will not have such weight as to prevent installers from handling it manually during installation. It is essential that a new chamber be sufficiently strong, in resisting the weight of overlying soil (typically largely crushed stone), any pavement surfacing and any motor vehicles or the like which traverse the pavement.
Buried corrugated plastic pipe has been used for a longer time than storm chambers and there is a developed technology for engineering design and analysis of such. See Section 12.12 “Thermoplastic Pipes” in “AASHTO LRFD Bridge Design Specifications—U.S. Units, 2003 Interim Revisions,” published by Amer. Assoc. of State Highway and Transportation Officials (AASHTO), Washington, D.C., Code LRFDUS 2-15 (April 2003). See also NCHRP Report 438 “Recommended LRFD Specification for Plastic Pipe and Culverts” published by Transportation Research Board of National Research Council, National Academy Press, Washington, D.C. (2000). However, whereas pipes have circumferentially continuous cross sections, chambers have open bottoms and free opposing side bases. Thus, chambers behave differently and the specifications, design criteria and modes of evaluating behavior which have been developed for pipe have to be adapted to chambers. An objective of the present invention is to provide large stormwater chambers which have performance and safety factors consistent with those achieved with corrugated plastic pipe.
Another criterion that is important for old and new chambers relates to economical shipping and storage. For that, chambers must nest well one within the other. Thus, for example, a desire for certain strengthening features, such as ribs or such as corrugations which are closely spaced with steep sides, can conflict with the need for good nesting.
As is well known, engineers have to be careful when scaling up the size of products, since what previously might have been minor design factors can become critical factors. Obviously, if chamber width is increased, more overlying weight is supported by the chamber, and strength must be sufficient. One way of increasing strength in a chamber is to increase the thickness of the chamber sidewalls, sufficient to reduce stress so it is within design criteria. But doing that has substantial disadvantages, as follows.
Commercially feasible chambers have to be fabricable by economic mass production means. Injection molding is the only practical way to fabricate a chamber with carefully controlled thickness dimensions. However, if an injected molded chamber is made with substantially varying wall thickness, problems arise with respect to mold filling and distortion of the part during cooling after removal from the mold. Thus, experience has shown that a practically-manufactured chamber should have substantially uniform wall thickness. But if wall thickness is uniformly increased to provide sufficient capability to the strength-limited regions of the chamber, the resultant chamber may have an undesirably increased weight and attendant material cost. Furthermore, the injection capacity limits of commercially available injection molding machines may be reached, limiting choice of vendors or making injection molding impossible. Thermoforming is an alternative way for forming chambers, but the nature of the process is such that unwanted thin areas will be present in the product, due to the stretching of the sheet being formed into the chamber. That can mean that, in order to achieve a minimum required dimension at a particular point, a larger than needed thickness has to be accepted in other less-stretched areas, with resultant uneconomic use of material.
In the alternative, internal or external ribbing can provide good strength. However, such ribbing tends to increase the stacking height, that is, the vertical spacing between two nested chambers. Ribbing can also introduce molding problems. In recent years, commercial favor has been given to stormwater and leaching chamber designs have smooth curve cross sections and which avoid significant ribbing.
Thus, there can be complicated tradeoffs in the design of a chamber, necessary to best attain all the competing aims. Any new larger chamber must be economical to make in terms of the amount and cost of plastic, the cost of manufacturing, and cost of shipping. In such context, there is a need for chambers which are larger than heretofore, which are practically fabricated, transported, and stored, and which in use have good strength on a short term and long term basis. Chambers are typically interconnected as strings. The ends of the strings must be closed off by end caps to prevent the surrounding crushed stone aggregate or other medium from entering the concave space under the chamber. Heretofore caps used with storm chambers and with leaching chambers have comprised flat plate and dome shape closures, typically with heavy ribbing. There is a need for improvements in end caps in the same general way as there is need for improved chambers.